Surveillance sensor which is provided with at least one surveillance radar antenna rotatable about at least one first axis of rotation

ABSTRACT

The invention relates to a surveillance sensor provided with at least one surveillance radar antenna 1 and at least one co-located and co-rotating electro-optical surveillance sensor 5 mechanically connected to said radar antenna. A combined panoramic picture is compiled by combining information from both sensors using a common track unit 30.

BACKGROUND OF THE INVENTION

The invention relates to a surveillance sensor which is provided with atleast one surveillance radar antenna rotatable about at least one firstaxis of rotation.

An embodiment in which such a surveillance radar antenna rotates about avertically oriented axis of rotation at a fixed rotation rate, has thedisadvantage that the information obtained by such an antenna is limitedto azimuth and range data of the detected objects. However, for trackinga moving object by means of a surveillance radar antenna or forcontrolling a tracking radar which is to track an object detected by thesurveillance radar antenna, it is advantageous to have the disposal ofadditional elevation data of the object. Indeed, additional elevationdata may be obtained by a so-called phased-array surveillance antenna,but the disadvantage of this antenna is that it is expensive andcomplex. An further disadvantage is that even a radar, capable ofproviding information concerning the speed of detected objects on thebasis of the Doppler effect, has trouble detecting stationary or slowlymoving objects in a high-clutter environment, e.g. helicopters aboveland.

SUMMARY OF THE INVENTION

The present invention has for its object to provide a surveillancesensor which obviates the said disadvantages and which is characterisedin that the surveillance sensor is provided with at least oneelectro-optical sensor mechanically connected to the surveillance radarantenna.

The azimuth and elevation information for a relatively inexpensiveelectro-optical sensor can now be combined in a simple way with theazimuth and range information from a radar sensor. The combinationobviates any problems caused by mutual alignment and parallax of thelines-of-sight of two separate sensors placed at a certain distance fromeach other.

When positioned on a ship, the alignment between two separate sensorswith a certain distance between them, may fluctuate due to distortion ofthe ship. The parallax taken as the difference in the angle at whichboth sensors observe a target, is now fractional, while the alignment issimple and stable. The mutual distance between two sensors of 5 metresgives a maximum parallax of 5 mrad in angle for an object at a 1 kmdistance, which parallax amply exceeds the resolving power of anelectro-optical surveillance sensor which can amount to 0.5 mrad.

Moreover, both costs and space are saved because only one commonstabilisation system is used.

By combining the information from two sensors, active at a differentwavelength range, the detection of stationary objects in high-clutterenvironments is improved. Indeed, the target contrast may be high due toe.g. reflections or heat radiation from hot engine parts in the visibleor infrared range, while the contrast in the radar wavelength range islow. Moreover, an electro-optical sensor can be used to select objectsof particular expected dimensions, thus providing an additional aid indistinguishing targets from their background.

It should be noted that radar trackers exist which are assisted indetecting a target by, e.g. an infrared surveillance sensor. Afterdetection of a target by an infrared surveillance sensor, the radartracker takes over the target and keeps tracking it by continuouslypointing a track antenna at the target. However, a radar tracker of thiskind cannot be used to give an overall picture of the surroundings,contrary to a surveillance radar, which for this purpose periodicallyrotates an antenna about a usually vertical axis at a customary rotationtime of at least several seconds required for sufficient illumination ofa target.

Moreover, infrared surveillance equipment exists which provides apanoramic picture of the surroundings by also rotating about a verticalaxis. Rotation times up to approximately half a second are customary forthis equipment. Owing to the usually high resolution of an opticalsensor, lower rotation times may lead to blurring in case of positioningon a moving platform. A combination of both types of surveillancesensors is, for this reason amongst others, not obvious, because itwould lead to a degradation in properties of one or both surveillancesensors.

Patent application EP 0.205.794 discloses a cooperation between a radarapparatus and an infrared surveillance device. However, this inventiondoes not yet concern an integrated design of an infrared sensor with aradar antenna, but is based on any known radar antenna and anindependently operating infrared surveillance device mounted separatelybeside the radar antenna.

This combination has all the disadvantages relating to mutual alignmentand parallax as discussed above. Besides, the radar antenna is notassigned any independent task, but is only used to verify the detectionsof the IR surveillance device.

In this case, the problem relating to different rotation rates of thetwo systems is obviated by not integrating the two units mechanically,in contrast with the present invention.

An advantageous embodiment is characterized in that the electro-opticalsensor is rotatable about the first axis of rotation. This results in awide azimuth range of the electro-optical sensor.

Another advantageous embodiment is characterized in that theelectro-optical sensor is so rigidly connected to the surveillance radarantenna that the rate of rotation about the first axis of rotation isthe same for the electro-optical sensor and the surveillance radarantenna. The rigid connection provides a stable alignment and obviatesthe need to provide separate rotation means for the electro-opticalsensor.

Another advantageous embodiment is characterized in that thesurveillance radar antenna is provided with rotation drive meanssuitable for rotating the electro-optical sensor about the first axis ofrotation with respect to the surveillance radar antenna. In this case,the electro-optical sensor may rotate at a higher rate to obtain data ata higher frequency. On the other hand, a lower rate of rotation canincrease the sensitivity of the electro-optical sensor.

A low pulse momentum of the surveillance antenna is obtained in anembodiment characterised in that the electro-optical sensor is connectedto the surveillance radar antenna in such a way that the electro-opticalsensor is mounted practically in line with the first axis of rotation.

An alternative embodiment of the surveillance sensor is characterised inthat at least one line of sight of the electro-optical sensorpractically coincides with at least one line of sight of thesurveillance radar antenna. The advantage of this construction is thatthe information from the radar antenna and from the electro-opticalsensor can be correlated instantly.

An embodiment characterised in that at least one line of sight of theelectro-optical sensor is directed opposite to at least one line ofsight of the surveillance radar antenna has the advantage that an objectis observed by one of the two sensors at a higher repetition frequency.

An embodiment characterised in that the electro-optical sensor isprovided with detector elements arranged in a row, which detectorelements form a line, parallel to the first axis of rotation, of thefield of view covered by the electro-optical sensor has the advantagethat no mechanical scanning means are required for the electro-opticalsensor.

Furthermore, an advantageous embodiment of the surveillance sensor ischaracterised in that the detector elements are sensitive to infraredradiation. The advantage of this embodiment is that the electro-opticalsensor can also be used at night.

Another embodiment of the surveillance sensor, comprising specialadvantages, is characterised in that

the electro-optical sensor and surveillance radar antenna are adjustablewith respect to each other across a limited angle about a second axis ofrotation,

the surveillance sensor is provided with adjusting means suitable forexecuting the said adjustment, and

the second axis of rotation is oriented practically perpendicular to thefirst axis for rotation.

A wide elevation range is thus obtained, where the electro-opticalsensor and the radar antenna each illuminate a section of that elevationrange. In case the radar antenna has a wider elevation range than theelectro-optical sensor, the latter may be adjusted in elevation, ifnecessary per revolution.

An alternative embodiment having the same advantage as the one describedabove is characterised in that the electro-optical sensor is providedwith at least one optical deflection element suitable for deflecting anoptical line of sight of the electro-optical sensor across a limitedangle about a third axis oriented perpendicular to the first axis ofrotation.

When the sensor is mounted on a moving platform, orientationstabilisation will be advisable. A first embodiment is theretocharacterised in that the surveillance radar antenna is provided withfirst orientation stabilisation means suitable for perpendicularlyorienting the first axis of rotation with respect to an earth-orientedreference plane. In this construction the entire surveillance sensor isstabilised.

A second embodiment is characterised in that the surveillance radarantenna is provided with beam-orientation adjusting means. In thisembodiment, control of the beam orientation results in a stabilisedsweep. Because the need for an expensive and heavy stabilisation unitfor the surveillance radar antenna is now obviated, the advantage ofthis embodiment is that the surveillance sensor remains light-weight andinexpensive.

A further advantage is that the stabilisation requires only a very lowtime constant, because mass inertia effects of the mass of the entiresurveillance sensor do not play a part.

A third embodiment is characterised in that the secondorientation-stabilisation means comprise a mirror adjustable in angleand mounted on the optical axis of the electro-optical sensor. Separatestabilisation of the electro-optical sensor reduces blurring of thepicture recorded.

An advantage of an application of an electro-optical sensor relates tothe undetectability of the sensor due to its passive character. Toretain this advantage in a combination with a radar it is advisable tolink the surveillance antenna with a transmitter generatinglow-intensity radar radiation. A favourable embodiment of a radar systemis characterised in that the surveillance radar antenna is provided withan FM-CW transmitting and receiving device.

The advantages of combination are increased in an embodiment where thesurveillance sensor is provided with a radar plot extractor connected tothe surveillance radar antenna for obtaining each antenna revolution atleast azimuth, range and doppler speed information; with anelectro-optical plot extractor connected to the electro-optical sensorfor obtaining at least azimuth and elevation information each revolutionof the electro-optical sensor; and with a common track unit connected tothe radar plot extractor and the electro-optical plot extractor forcombining the information obtained, for generating on the basis of thecombined information a target track to control a weapon system to beconnected. By combining the information originating from both sensors inthe earliest stage possible, and subsequently supplying it to a commontrack unit, a considerable amount of time is gained in comparison with aconfiguration comprising a separate track unit for each sensor.

In an embodiment where the track unit comprises a fast logic unit forpromoting combined information to a target track within the timerequired for one revolution of the surveillance sensor, if thisinformation complies with preset criteria, the normal track process maybe reduced for urgent situations.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be explained with reference to the accompanyingfigures, of which:

FIG. 1 represents the surveillance radar antenna provided with anelectro-optical sensor;

FIG. 2 is a schematic diagram of a rotation-fixed embodiment of theattachment of the electro-optical sensor;

FIG. 3 is a schematic diagram of a rotatable embodiment of theelectro-optical sensor; and

FIG. 4 is a schematic diagram of the attachment of the surveillanceradar antenna on a platform.

FIG. 5 shows a signal processing unit for the combination of the signalsfrom the surveillance radar antenna and the electro-optical sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment according to the invention is shown in FIG. 1. Asurveillance radar antenna 1 is rotatable with respect to an antennasupport 2 about a first axis of rotation 3 by means of drive means 4. Inthis embodiment, surveillance radar antenna 1 is a well-known slottedwaveguide. An electro-optical sensor 5 is fixed in line with the firstaxis of rotation 3 on surveillance radar antenna 1 by means of fixingmeans 6. The electro-optical sensor 5 is rotatable about a second axisof rotation 7 oriented perpendicular to the first axis of rotation 3.

A first feasible embodiment of fixing means 6 is shown in FIG. 2. Theelectro-optical sensor 5 is suspended in a supporting frame 8 which isfixed to a top side 9 of surveillance radar antenna 1. Theelectro-optical sensor 5 is rotatable about a second axis of rotation 7,which is oriented perpendicular to the first axis of rotation 3, and isthereto provided with suitable bearing 10, rotation drive meanscomprising a gear transmission 11 and 12 which are linked with servomotor 13. The optical line of sight of electro-optical sensor 5 in thisembodiment crosses the first axis of rotation 3. A feed-through aperture14 is provided for feeding through electrical and/or optical signalcarriers to and from the electro-optical sensor 5.

FIG. 3 shows an embodiment of a rotatable connection of theelectro-optical sensor 5 to the surveillance radar antenna 1. Frame 8,in which the electro-optical sensor 5 is suspended, is fixed to asupporting part 15 which is supported by a bearing 16. This bearing 16allows rotation of the supporting part 15 about an axis of rotation 17with respect to top side 9 of surveillance radar antenna 1. In thisembodiment, this axis of rotation 17 coincides with the first axis ofrotation 3, but axis of rotation 17 may also run parallel but notcoincidental with the first axis of rotation 3. Gear transmission 18 andservo motor 19 are applied for this rotation. To allow feed-through ofelectrical and/or optical signal carriers, feed-through aperture 14 isprovided, as well as a well-known rotatable electrical and/or opticalcoupling 20.

FIG. 4 is a schematic diagram of the attachment of radar antenna 1 toantenna support 2. Of the radar antenna, only a waveguide 21 to aslotted waveguide 22 with beamforming plates 23 is depicted. Thesebeamforming plates are adjustable in elevation angle, allowing controlof the antenna orientation in elevation. This allows a stabilised sweepof the beam axis. In an embodiment comprising a radar with electronicbeamforming, the same effect can be obtained by means of electroniccontrol of the beam orientation. In this embodiment, frame 8 in whichthe electro-optical sensor 5 is suspended is rigidly fixed to top side 9of surveillance radar antenna 1. By means of a bearing 24, radar antenna1 is able to rotate with respect to antenna support 2 about axis ofrotation 3. For this purpose, a gear transmission 25 and a servo motor26 are applied. The rotatable electric and/or optical coupling 20function as a rotatable coupling between electric and/or optical signalcarriers to electro-optical sensor 5. Besides, a rotary waveguidecoupling 27 is applied for waveguide 21.

Depending on the mechanical construction of radar antenna 1, theelectro-optical sensor 5 can also be fixed in other positions, e.g. inline with the radar beam axis 28 on the back of the antenna. Analternative embodiment is obtained by fixing the electro-optical sensor5 inside the cover of radar antenna 1.

Electro-optical sensor 5 is preferably an infrared-sensitive sensorwhich is also capable of functioning at night and under adverseatmospheric conditions. A more cost-effective embodiment is obtainedwith a daylight sensor such as a common TV camera or a imageintensifier.

A combination of the above-mentioned types of sensors has the advantageof increasing the detection probability because it enables selection ofthe sensor that offers the optimal target contrast.

The electro-optical sensor 5 is preferably a complete, autonomous cameraunit provided with the necessary optics, radiation-sensitive elementsand, if applicable, cooling equipment and control electronics forsampling and filtering of the received signals. To reduce blurring dueto movement to a minimum, the electro-optical sensor may be providedwith mirrors or prisms adjustable in angle and positioned on the opticalaxis.

In case of the embodiment comprising the infrared sensor, a favourableembodiment is obtained when use is made of a line array of infraredsensitive elements, where the line array has a vertical orientation inthe field of view. The scanning movement perpendicular to thelongitudinal direction of the line array, required for picturecompilation, is obtained by means of rotation of the electro-opticalsensor 5 about the first axis of rotation 3. Furthermore, theelectro-optical sensor 5 can be supplemented with a laser rangefinderfor obtaining range information independent of the radar.

FIG. 5 shows the signal processing unit relevant to the combination ofsignals. In this signal processing part, signals originating from theelectro-optical sensor 5 are supplied to a plot extractor 29. In thisplot extractor 29 extraction of targets takes place by means of knownimage processing techniques. The azimuth, elevation and intensity valuesof possible targets obtained per full revolution of the rotating sensor5, are subsequently supplied to a common track unit 30. The signalsoriginating from surveillance radar antenna 1 are supplied to knownradar plot extraction means 31, which provide at least range, azimuth,signal strength and doppler speed information of possible targets. Thisinformation is also supplied per revolution of the antenna 1 to thecommon track unit 30. In case of a 3D surveillance radar antenna 1,elevation information is also added.

Track unit 30 comprises a fast decision unit 32 for the purpose ofselecting threatening situations in which the usual track processingwould take too much time. This is the case, for instance, when ahelicopter, appearing above the edge of a wood during a short time todetect a target, subsequently disappears to assign a weapon andreappears to fire the weapon. The usual track processing would in thatcase take too much time to allow target engagement within the short timethe helicopter is visible. Fast decision unit 32 therefore selectstargets with a Doppler speed of practically zero and an elevation aroundthe visible horizon to supply this information to a fast track unit 33which, preferably within the time required for one revolution, generatesa track for the purpose of control of weapon system 34. A track in thiscase is understood to be target position information related to a targetwithin a fixed coordinate system and during several measurements,suitable to control a weapon system. It should be noted that in the caseof surveillance radar, it is customary to only generate a track afterseveral revolutions of the antenna, which would in this case take toomuch time. However, the available elevation information and simultaneousdetection in another wavelength range as a result of the combinationwith an electro-optical sensor may produce a reasonably reliable trackafter only one revolution.

In non-threatening situations, the information from both sensors issupplied to a normal track unit 35. Here combination takes place of theinformation from both sensors on the basis of common azimuth values. Thecombined information is then mathematically expressed in a vector ofstate characterising the target, which vector is adapted eachmeasurement to new information from one or both sensors. Track unit 35uses well-known track algorithms with preset parameters dependent on theexpected target trajectory. For example, for a straight targettrajectory other parameters should be selected than for a curved targettrajectory. A target may have a sinusoidal trajectory of becometemporarily invisible. The latter occurs in the case of a helicopterpopping up every now and again from behind the edge of a wood.Well-known track algorithms are for instance α, β, γ algorithms, withparameters α, β and γ for weighting respectively the target position,target speed and target acceleration. It is important that the trackunit 35, at predetermined intervals, should generate track data tocontrol weapon system 34. Processing of the information originating fromextractors 29 and 31 should preferably be completed well within the timerequired for a next measurement. A process subject to strict time limitsis therefore also characterised as a real-time process. However, becauseof the variety of available information originating from sensorsoperating in different wavelength ranges and the complex environment inthe case of land-based applications, it is advisable not to limit thetrack processing to one algorithm only. For this reason, the trackprocess is supported by a non-real-time support unit 36. Support unit 36draws up hypothesis concerning the target and the target trajectory.With these hypothesis the parameters of the track algorithms can bedetermined. Within the support unit 36, a distinction can be madebetween functions for the generation of new hypotheses (37), formaintaining hypotheses (38), reflecting hypotheses (39) and for testingof hypotheses (40). New hypotheses may consist of the various targettrajectories mentioned before. Maintaining hypotheses implies that it isassessed whether the tracks suit the hypotheses selected.

Support unit 36 uses artificial intelligence technique in contrast tothe strict algorithmic track processes of track unit 30.

I claim:
 1. A surveillance apparatus comprising a surveillance radarantenna rotatable about a first axis of rotation for obtaininginformation representing the range and azimuth of a detected object andan electro-optical sensor rotatable about the first axis asynchronouslywith respect to the radar antenna for rotatably scanning azimuthsscanned by said radar antenna and obtaining additional informationrepresentative of at least one characteristic of the detected object,said surveillance apparatus further comprising:a. a radar plot extractorelectrically connected to the surveillance radar antenna for obtainingduring each revolution of said antenna at least azimuth, range anddoppler speed information; b. an electro-optical plot extractorelectrically connected to the electro-optical sensor for obtainingduring each revolution of said sensor at least azimuth and elevationinformation; and c. a common track unit selectively connected to theradar plot extractor and the electro-optical plot extractor forcombining the information obtained thereby and generating, on the basisof the combined information, information representative of a targettrack.
 2. A surveillance sensor as claimed in claim 1, characterised inthat the electro-optical sensor is mounted for rotation about the firstaxis of rotation relative to the surveillance radar antenna.
 3. Asurveillance sensor is claimed in claim 1, characterised in that theelectro-optical sensor is connected to the surveillance radar antenna insuch a way that the electro-optical sensor is mounted substantially inline with the first axis of rotation.
 4. A surveillance sensor asclaimed in claim 1, characterised in that the electro-optical sensor isprovided with at least one optical deflection element suitable fordeflecting an optical line of sight of the electro-optical sensor acrossa limited angle about a third axis oriented perpendicular to the firstaxis of rotation.
 5. A surveillance sensor as claimed in claim 1,characterised in that the surveillance radar antenna is provided withfirst orientation stabilisation means suitable for perpendicularlyorienting the first axis of rotation with respect to an earth-orientedreference plane.
 6. A surveillance sensor as claimed in claim 1,characterised in that the surveillance radar antenna is provided withbeam-orientation adjusting means.
 7. A surveillance sensor as claimed inclaim 6, characterised in that the beam-orientation adjusting meanscomprise plates adjustable in angle, which plates are positioned atleast in part of the outgoing beam.
 8. A surveillance sensor as claimedin claim 6, characterised in that the beam-orientation adjusting meanscomprise an electronic beamformer based on a phased array principle. 9.A surveillance sensor as claimed in claim 1, characterised in that theelectro-optical sensor is provided with second orientation stabilisationmeans.
 10. A surveillance sensor as claimed in claim 9, characterised inthat the second orientation stabilisation means comprise a mirroradjustable in angle and mounted on the optical axis of theelectro-optical sensor.
 11. A surveillance sensor as claimed in claim 9,characterised in that the second orientation stabilisation meanscomprise a cardan system in which the electro-optical sensor issuspended.
 12. A surveillance sensor as claimed in claim 1,characterised in that the information originating from the surveillanceradar antenna and the electro-optical sensor is combined on the basis ofcorresponding azimuth information.
 13. A surveillance sensor as claimedin claim 1, characterised in that the track unit comprises a fastdecision unit for effecting production from the combined information ofa target track within the time required for one revolution of thesurveillance sensor, if this information complies with preset criteria.14. A surveillance sensor as claimed in claim 13, characterised in thatthe fast decision unit effects production of a target track fromcombined information representing a target for which the Doppler speedis substantially zero and the elevation is substantially at a visiblehorizon.
 15. A surveillance sensor as claimed in claim 1, characterisedin that the track unit is adapted to generate target tracks atpredetermined time intervals by means of parametered track algorithmsand the surveillance sensor is provided with a track support unitconnected to the track unit for supplying the parameters for the trackalgorithms of the track unit at non-predetermined times.